1. Field of the Invention
The invention is directed toward a coating power for the electrostatic coating of glass as well as ceramic and metallic substrates; the main component of the coating powder is a powder-form material which forms a glass layer during firing. A further subject matter of the invention is directed toward a process for the production of the coating powder. A further subject matter of the invention relates to the use of the coating powder for glazing, slip painting and decorating glass and ceramiG'substrates as well as for enameling metallic substrates.
2. Description of Related Art
Among the materials for producing ceramic coatings, such as glazes, enamels and vitreous enamels on substrates capable of being fired, are understood to be glass as well as ceramic and metallic substrates, aqueous slips are largely used; following the application of the slip, the substrate coated with the slip is fired, in the process of which the material, capable of being fired in and comprised in the slip, melts together or is sintered to form a vitreous layer often also referred to as ceramic layer. Due to the disadvantages entailed in the use of aqueous slips, such as problems involving effluents and high energy requirements, the electrostatic powder coating gains in importance.
It is known that coating powders forming a vitreous layer during firing, can be sprayed electrostatically onto glass as well as ceramic and metallic surfaces. The electrostatically applicable coating powders must have a sufficiently high electric volume resistivity, customarily 10.sup.9 to 10.sup.14 .OMEGA..multidot.m. In order to attain the required resistivity value, the coating powders are coated with insulating substances, such as for example silanols and organopolysiloxanes, isocyanates and carbodiimides. By treating ceramic powders with polymethylhydrogensiloxanes, according to EP-A 0 382 003, apart from the required resistivity value, the adhesion of the powder on the ceramic surface can simultaneously be improved. Of disadvantage is that the production process for ceramic powders treated thus entails high expenditures.
WO 94/26679 teaches improving the adhesion and fastness to handling of an electrostatically applied glazing powder: in addition to a glazing powder, the coating powder in this case comprises an adhesion means which during the firing burns free of residues, such as polyolefins or dextrins which are activated thermally, respectively by moisture, in order to fix the particles of the layer with one another and with the substrate. Preferred coating powders comprise polysiloxane-coated glass frits in mixtures with 10 to 15 percent by weight of thermoplastic materials or 5 to 10 percent by weight of dextrin. Only under especially optimized conditions, which, however, lead to high expenditures, is it possible to obtain usable glazes on porcelain biscuit bodies. To some extent considerable glazing errors and faults occur already under slightly modified conditions as a function of the substrate before the required layer thickness is reached.
An improvement of said problems is taught by WO 97/08115 when using a glazing or slip painting composition with a specific distribution of particle size, namely a d.sub.50 value of 5 to 25 .mu.m, a d.sub.90 value of less than 35 .mu.m and a d.sub.10 value equal to or greater than 2 .mu.m, in a coating powder comprising additionally an adhesion means, it is possible to simplify the production process without loss of quality of the glaze.
WO 97/08115 also discloses that in the presence of an organopolysiloxane the electric resistivity of the coating powder increases in the desired way with increasing quantity of the adhesion means. Above a specific quantity of adhesion means, however, spray-back effects occur such that the required layer thickness can no longer be applied so that the fired-in layer is too thin and accordingly has flaws.
One problem of electrostatic coating of glass and ceramic substrates comprises also that the coating powders have highly differing volume resistivities as a function of their composition and their range of particle size. Added to this is the fact that the substrates to be coated have a highly varying surface conductivity which, additionally, depend on the climate conditions in the coating chamber. Fault-free electrostatic coating presupposes that the volume resistivity of the coating power is matched to the substrate to be coated and to the climate conditions. While previously the highest volume resistivity possible of the coating powder was demanded, it has been found in the meantime that very high resistivity values are indeed useful with low layer thickness, for example with thicknesses of 10 to 80 .mu.m, conventional for glass decorations, however, that with higher layer thicknesses, for example 100 to 400 .mu.m, such as are demanded for glazes for tiles, spray-back effects develop due to too high a resistivity.